Première contribution à l'étude des évolutions paléohydrologiques et à l'histoire des feux en Champagne berrichonne durant l'Holocène. Le cas du “Marais du Grand-Chaumet” (Indre, France)

Les plateaux de basse altitude constituent des espaces géographiques peu documentés. Le Berry (Centre de la France) confirme cette réalité de la recherche avec plus particulièrement la Champagne berrichonne qui correspond à un vaste plateau calcaire aujourd'hui quasiment vierge de toute étude paléoenvironnementale. La mise en place d'un programme de recherche sur l'occupation du sol dans la cité des Bituriges Cubi, a permis de développer une approche paléoenvironnementale dans cette zone en réalisant un effort méthodologique particulier en réponse aux difficultés de terrain. L'objectif poursuivi est d'établir un schéma général des évolutions paléoenvironnementales durant l'Holocène par une reconstruction des dynamiques paléohydrologiques et de l'histoire des feux. L'étude de la séquence sédimentaire du “Marais du Grand-Chaumet” (Indre) permet de mettre en évidence l'occurrence importante de feux durant la première partie de l'Holocène suggérant une place non négligeable de ceux-ci dans l'interprétation et la compréhension des évolutions environnementales au cours de cette période. L'histoire des incendies, corrélée aux reconstitutions paléohydrologiques, montre pour l'Holocène une évolution des conditions environnementales complexes. Il en ressort également un caractère abrupt des évolutions environnementales parfaitement en accord avec l'idée qu'on peut se faire d'un événement d'incendie. Ces évolutions ont certainement eu un impact important sur le développement des sociétés

Evolution of organic matter indicators in response to major environmental changes: the case of a formerly cutover peatbog (Le Russey, Jura Mountains, France).

To assess whether the biochemical characteristics of peat can provide clues for past ecosystem changes or not, a study was carried out combining elemental analysis, micro-morphological counts and neutral monosaccharide determination of peat organic matter (OM) and the dominant living plants from a formerly cut-over peat bog in the Jura Mountains. Peat profiles (up to 50 cm depth) from two distinctive zones (bare peat, FRA and a regenerating stage, FRC) were compared with the reference profile (FRD) taken from an unexploited area of the bog. The results show contrasting OM composition along the profiles. In the upper sections of the FRD and FRC profiles, high C/N ratios and sugar contents (in the same range as in the source plants) and the large predominance of well preserved plant tissues indicate good preservation of primary biological inputs. In contrast, in peat from the FRA profile and deeper levels of the FRC profile, lower C/N ratios, lower amounts of sugars and a predominance of amorphous OM and mucilage suggest more extensive OM degradation. These features delineate a clear threshold between an uppermost "new" regenerating peat section and an "old" catotelm peat below. Nevertheless, even in the latter, the sugar contents remain relatively high (>80 mg/g) compared with other organic and mineral soils. Analysis of typical peat-forming plants and of bulk peat and fine grained fractions allowed identification of the following source indicators: xylose and arabinose for Cyperaceae; rhamnose, galactose and mannose for mosses; and ribose (and to a lesser extent, hemicellulosic glucose) possibly for microbial synthesis

A water-table dependent reservoir model to investigate the effect of drought and vascular plant invasion on peatland hydrology.

International audienceThis paper investigates the water table dynamics in a peatland showing a wide range of water table fluctuations. A reservoir model of water table fluctuations in a double-porosity peat is proposed, by calculating the stored water in effective porosity of the peat from precipitation and evapotranspiration datasets. Calculations conceptualize vascular plant consumption through a crop coefficient. Changes in water storage, located in the effective porosity of the peat, are described through a maximum infiltration rate and a maximum storage capacity. Water discharges take place in runoff and percolation reservoirs. The runoff coefficient is considered to be water table dependent. This model was tested on a peatland that has experienced strong water table fluctuations caused by summer drought and/or by vascular plant water consumption. A water table dependent runoff model appeared to be adequate to describe the water table fluctuations in peatland. From this model, vascular plants were found to increase the crop coefficient and to limit percolation through the peat. The high water table depth in winter was found to change with the year and is related to an equilibrium between slow infiltration in peat versus percolation plus evapotranspiration. In this disturbed peatland, even if overland flows occurred after a drought, the re-saturation of effective porosity was slow with about 30% of air trapped in the porosity 6 months after the drought period. The effects of drought on peat saturation were observed over more than a single hydrological cycle. This can affect the biogeochemical processes controlling the C cycle in peatland

Multi-Pluridisciplinary approach of the evolution of the agro-pastural activities in the surroundings of the " narse d'Espinasse " (French Massif Central, Puy de Dôme).

The ldquonarserdquo or peat marsh of Espinasse (Saulzet-le-Froid district) situated in the southern part of the Chaîne des Puys has been the subject of a new pollen analysis concentrating on the anthropogenic impact on vegetation evolution since the Sub-Boreal. Human occupation of the surroundings of the narse is dated as early as the Neolithic, which is usual for the region. There is nevertheless an isolated record of Fagopyrum related to the Neolithic. This is a unique occurrence in the Massif Central. For successive periods and up to the recent past, a dynamic of various anthropization phases has been reconstructed. The combination of palynological data with archaeological and historical sources has for certain periods, mainly from the 11th to 13th centuries, provided new insights on the social and technical management of the territory. Furthermore, geochemical and micromorphological characterisation of sedimentary organic matter has led to the identification of erosive crises and silting which would have followed massive tree cutting in the region. On the local scale, the highly degraded organic matter at the top of the peat profile is the consequence of the current drainage of the marsh

Micromorphological and (bio)chemical organic matter changes in a formerly cutover peat bog : Le Russey, Jura Mountains, France.

Peatlands have been exploited since the Middle Ages for diverse uses, including fuel, animal bedding and growth substrate in horticulture and agriculture. In many countries, these traditional peatland uses are now redundant, but the sites are being reappraised for their specific biodiversity and suitability for long-term carbon storage

Coalbed methane producibility from the Mannville coals in Alberta, Canada: A comparison of two areas

International audienceThe Mannville coals in the Fenn area, Alberta Plains, have desorbed gas content averaging 8.57 cm3/g (275 scf/t), which is similar to the same coals in the Corbett Creek area, almost 400 km away. Vitrinite reflectance values are also similar, although the coals at Corbett Creek are situated about 300 m shallower, which points to a rank excursion from Hilt's burial law curves at Corbett Creek. Coals from both areas are within the “oil window”. The Medicine River Seam in the Fenn area has higher total inertinite content and greater proportions of inertodetrinite and detrovitrinite, suggesting that peat deposition occurred in swamps and marshes and were prone to periodic flooding. At Corbett Creek, the Mannville coal seams are characterized by greater concentrations of telo-inertinite, which contributes to coal meso-porosity and the potential for free gas storage in the open cell lumens, and to an increased gas flow along lithotype boundaries (horizontal permeability). Non-fluorescing vitrinite was present mostly in the upper Medicine River Seam, which was deposited in a regressive environment. The lower Medicine River Seam, which formed during a marine transgressive phase, contained greater amounts of fluorescing vitrinite. The Mannville coals in the Fenn area are moderately under-pressured in relation to those at Corbett Creek, which may have an impact on gas retention capacity. The difference in absolute coal permeability (1-3.5 mD at Fenn versus 3-4 mD at Corbett Creek), which is likely the result of higher in-situ stresses in the deeper Mannville coals at Fenn, has had an effect on both gas and water production rates from these coals. However, the largest impact on gas production volumes has been made by the application of horizontal drilling technology, initially at Fenn, and more recently by multiple horizontal wells drilled at Corbett Creek

Invasion of a Sphagnum-peatland by Betula spp and Molinia caerulea impacts organic matter biochemistry. Implication for carbon and nutrient cycling.

Article AcceptedInternational audiencePeatland act as a sink of carbon (C) through the conservation of dead remains of plants. Under global changes triggered by human activities, it is not only the sink capacity of peatland that is in danger, but also the C already stored. Invasion of Sphagnum peatlands, mainly by Molinia caerulea and Betula spp, is a growing preoccupation. This study aims to assess the extent of the influence of such invasion on the biochemical characteristics of the peat. Elemental analysis and Rock Eval pyrolysis parameters were measured in 50 cm profiles collected in invaded and intact plots. The results showed that Oxygen Index (OI) can be used to detect new C substrate injection as such new materials have a lower OI than Sphagnum spp and Sphagnum peat. Differences in terms of water table level and nutrient status are suggested to be of great importance to determine the fate of surface and deep new C input. This study stimulates new investigation testing a possible priming effect triggered by the invading vegetation and using stochiomeric constrain as a theoretical framework

Study of physical and chemical properties of vitrinites. Inferences on depositional and coalification controls

URA. 724 du CNRS a été intégrée dans l'UMR 6113 - CNRS, Université d'Orléans : ISTOInternational audienceA detailed study of petrological, geochemical, textural and coking properties was carried out on vitrains from the Puertollano, Blanzy–Montçeau, Asturias and Teruel. The objective was to determine in depth the physical and chemical properties of a series of natural and pure vitrinites of different rank, and the influence that the sedimentary and post-sedimentary conditions had on them. It is demonstrated that although vitrains are almost entirely made up of the huminite/vitrinite maceral group they have a different composition, thermal behaviour and physical properties. Thus, geochemical and textural properties of Blanzy–Montçeau vitrains can be considered to be representative of the telinite, the major component in both samples (75% vol.) at the beginning of the bituminization stage (subbituminous/high volatile C bituminous coals). The characteristics of the Puertollano vitrains described here can also be attributed to the telocollinite (>80% vol.) for the high volatile C bituminous coal. Variations in physico-chemical properties between Puertollano and Blanzy–Montçeau vitrains are due to the differences in the initial composition of organic matter. Slight differences (i.e., bed moisture content or porosity) between vitrains from the two coal seams in the same basin can be attributed to their stratigraphic position. Several parameters such as S2, HI, oil and extraction yield and fluorescent properties suggest that the Puertollano and Blanzy–Montçeau vitrains have a lower hydrocarbon potential. The relationships between geochemical and textural properties make it possible to distinguish between ‘normal' and perhydrous vitrains. The two different hydrogen-enrichment processes that occur in vitrains from the Teruel and Asturias basins can be distinguished from the extraction yield data

In situ quantification of CH4 bubbling events from a peat soil using a new infrared laser spectrometer

International audienceCH4 emissions from peatlands are space- and time-dependent. The variety of efflux routes contributes to these variabilities. CH4 bubbling remains difficult to investigate since it occurs on a timescale of seconds. The aims of this study were to test the capacity of a recently built infrared high resolution spectrometer, SPIRIT (SPectrometre Infra-Rouge In situ Troposphérique), to (1) measure in situ CH4 fluxes, (2) observe online bubbling events with quantification of CH4 emission fluxes corresponding to this very sudden degassing event. Material and methods: The closed dynamic chamber method was used and the gas concentration was measured every 1.5 seconds. Emission fluxes were calculated by the accumulation rate of CH4 against time. Measurements were undertaken during daytime in March 2009 and during day- and nighttime in May 2009, in a bare peat area, temporarily forming a shallow pool. Results and discussion: The results show that the CH4 emissions estimated with the SPIRIT ranged from 2.79 to 86.0 mg CH4-C m-2 h-1. These values are consistent with those already published. The high emissions, both through diffusion and bubbling, were on the same order as the emissions estimated in natural shallow pools. During daytime, CH4 bubbling was higher in May (56.2% of the total emission) than in March (40.7%) probably because of increased CH4 production and accumulation in peat. In May, bubbling was higher at nighttime (68.6%) than in daytime (56.2%). This has an important implication for carbon budget assessment in peatlands, particularly in boreal areas. Conclusions: The recently built infrared spectrometer, SPIRIT, was able to reliably measure CH4 fluxes and quantify CH4 flux during the degassing of a bubble. The emissions obtained are in agreement with previously published data using other measurement techniques. The results of this preliminary work highlight (1) the importance of shallow pools in peatland CH4 emissions, (2) the sensitivity of such fluxes to atmospheric pressure, a relation that has not been fully investigated or taken into account in assessing peatland carbon balance

Sequential fatty acid analysis of a peat core covering the last two millennia (Tritrivakely lake, Madagascar): Diagenesis appraisal and consequences for palaeoenvironmental reconstruction.

Seven samples from a 1 m long peat core, taken from the centre of the Tritrivakely maar lake (Madagascar) and covering the last 2300 yr, were analysed for their fatty acid (FA) content. Solvent extraction, followed by acid hydrolysis and saponification allowed successive release of “free”, H+-labile and OH−-labile bound FAs. Downcore variations in the concentration of individual compounds released by the different treatments allowed distinction between the following three major types of input: (i) primary production; (ii) secondary production (that most certainly forms as a result of microbial degradation of primary products) immediately underlying the sub-surface; (iii) a notable bacterial stock that appears later and slowly at depth. The latter input is revealed by the regularly increasing concentration of several H+-labile compounds with depth. Sedimentary horizons that were sites for good preservation conditions are indicated by high relative amounts of nearly all the compounds, especially those inherited from the primary as well as from the secondary producers (including the primary consumers). In contrast, depth intervals that have been the site for intensive diagenetic alteration contain few compounds, including those inherited from late consumers